1. Field of the Invention
The present invention relates to an optical pickup apparatus for recording information on an optical recording medium or reproducing information from an optical recording medium.
2. Description of the Related Art
FIG. 13 is a view schematically showing a configuration of a conventional optical pickup apparatus 1. An optical disc 7 serving as an optical recording medium has a recording layer 8. When information is recorded and/or reproduced on and from such an optical disc 7, laser light is collected on the recording layer 8 of the optical disc 7 and the recording layer 8 is irradiated therewith, and then reflected light from the optical disc 7 is received by a light receiving portion 9. In this way, the information is recorded and/or reproduced, and servo control is carried out by detecting servo signals including a focusing error signal and a tracking error signal.
In the conventional optical pickup apparatus 1, light emitted from a light source 2 is split into a main light beam and a pair of sub light beams by a grating element 3, and which are transmitted through a hologram element 4, are collimated by a collimation lens 5, and thereafter are collected on the recording layer 8 of the optical disc 7 by the objective lens 6. The reflected light from the recording layer 8 of the optical disc 7 is transmitted through the objective lens 6 and the collimation lens 5, and the light thus transmitted is diffracted by the hologram element 4 and thereafter is received by the light receiving portion 9 in a state of being collected. Such a conventional optical pickup apparatus 1 as described above is disclosed in Japanese Unexamined Patent Publication JP-A 2004-303296.
FIG. 14 is a view showing a relation between the hologram element 4 and the light receiving portion 9. The light receiving portion 9 has a light receiving portion for focusing 11 that is used for generating a focusing error signal, and a pair of light receiving portions for tracking 12a and 12b that are used for generating a tracking error signal. The hologram element 4 has a diffracting portion for focusing 13 that is used for diffracting the reflected light from the recording layer 8 of the optical disc 7 toward the light receiving portion for focusing 11, and a pair of diffracting portions for tracking 14a and 14b that are used for diffracting the reflected light from the recording layer 8 of the optical disc 7 toward the respective light receiving portions for tracking 12a and 12b. 
The optical disc 7 may have a plurality of recording layers 8 laminated in its thickness-wise direction for the purpose of increasing a recording capacity. In recording and/or reproducing the information on and from such an optical disc 7, the light receiving portion 9 not only receives reflected light from a light-collecting recording layer 8a which is currently involved in collection of the light emitted from the light source 2, but also undesirably receives reflected light from a non-light-collecting recording layer 8b that is another layer than the light-collecting recording layer 8a. 
FIG. 15 is a view for explaining an overview of transmittance and reflection of light in the optical disc 7 having two recording layers 8. In FIG. 15, it is assumed that the non-light-collecting recording layer 8b is provided behind the light-collecting recording layer 8a when the optical disc 7 is viewed from a side where the light source 2 is located.
Light 16 emitted from the light source 2 is collected onto the light-collecting recording layer 8a and is reflected thereon. At this time, a part of the light 16 emitted from the light source 2 is transmitted through the light-collecting recording layer 8a. Transmitted light 17 that has been transmitted through the light-collecting recording layer 8a is reflected by the non-light-collecting recording layer 8b. In this case, the reflected light from the non-light-collecting recording layer 8b reaches the respective light receiving portions for tracking 12a and 12b, and is received undesirably by the respective light receiving portions for tracking 12a and 12b, with its diameter of light beam greater than that of the reflected light from the light-collecting recording layer 8a. As a result, this causes a problem of adverse effects on the tracking error signal.